The production of microcircuits requires, among other steps, the repeated application of photolithographic etching The photolithographic process includes coating the surface of the semiconductor wafer with a thin and uniform film of photosensitive emulsion (photoresist). A photomask is then placed adjacent the film and the system is exposed to UV radiation. Polymerization of the photosensitive emulsion is produced through the mask at the transparent regions of the mask. After exposure, the photomask is removed and the wafer is immersed in a chemical developing bath which dissolves the non-polymerized parts of the photosensitive emulsion.
The photosensitive emulsion that has not been removed is then stabilized so that it protects the wafer when it is immersed in an acid solution which etches the SiO.sub.2 layer that is not protected by the emulsion. Finally, the remaining emulsion is removed with a chemical solvent and a mechanical abrasion process. Accordingly, photomasks are important in the process for manufacturing integrated circuits.
Standard-type masks are based, as mentioned, on the passage or blocking of light, i.e., an electromagnetic field. In other words, they are totally transparent in the light regions and totally absorbent in the dark regions. However, this entails that the obtainable resolution is relatively low. It is also necessary to note that the intensity of the light that reaches the wafer through the photomask is proportional to the square of the electromagnetic field.
Masks which are improved with regards to the obtainable resolution are Phase Shift Masks (PSM) which are not based on the absorption of light, but on a 180.degree. phase shift of the polarization of the electromagnetic field at areas that are not to be etched on the wafer. This is achieved by a "shifter" layer arranged on the mask. With these masks it is possible to achieve, as mentioned, a higher resolution with respect to standard masks and to achieve greater depth of focus on the surface of the wafer. Attenuated Phase Shift Masks (APSM) are also known which provide for partial absorption by the shifter layer.
Finally, the Embedded Attenuated Phase Shift Masks (EAPSM) are known to provide for the presence of an additional layer of surface chromium so that the shifter layer is embedded in the mask. The surface chromium layer is arranged in appropriate regions which must be protected, and must therefore be totally absorbent to prevent their double exposure to the light source from causing lithographic problems. The chromium layer blocks the light completely.
In masks of the EAPSM type, the chromium layer must therefore be present in regions where the mask provides for a double exposure due to the presence of two adjacent devices in regions where there are structures of the device that must be exposed to light radiation. The chromium layer must then be removed in accordance with process steps shown in FIG. 1, and as described in greater detail hereinafter.
The shifter layer is usually made of MoSi, MoSiON or other materials and has a transmittance of a few percent (usually 4 to 10%). FIG. 1 illustrates the standard process steps for manufacturing masks of the EAPSM type which require a double exposure. The first exposure defines the entire configuration, and the second exposure selectively removes the layer of chromium on the shifter layer, which in this case is made of MoSi.
In FIG. 1, the reference numeral 1 designates a quartz layer, the reference numeral 2 designates the shifter layer, the reference numeral 3 designates a chromium layer and the reference numeral 4 designates a photoresist layer. During step a), a first exposure/developing is performed and the layer of photoresist 4 is selectively etched.
Then, during step b), the chromium layer is etched, the photoresist layer is removed and cleaning is performed The subsequent step c) provides for etching of the shifter layer 2 at areas that are not covered by the chromium layer 3. In step d) a new photoresist layer is applied to the chromium layer 3, and a second exposure is performed to remove the chromium. Step e) provides for the etching of the chromium layer 3 and for the removal of the photoresist layer to obtain the structure shown at step e) in FIG. 1.
The method for manufacturing masks of the EAPSM type described above provides for a removal of the chromium layer according to the individual device to be drawn, i.e., a removal which is performed on a case-by-case basis and cannot be standardized.